Carbamate-functional and urea-functional materials have found particular utility in coating compositions as crosslinkable resins. Curable coating compositions utilizing carbamate-functional resins are described, for example, in U.S. Pat. Nos. 5,693,724, 5,693,723, 5,639,828, 5,512,639, 5,508,379, 5,451,656, 5,356,669, 5,336,566, and 5,532,061, each of which is incorporated herein by reference. These coating compositions can provide significant advantages over other coating compositions, such as hydroxy-functional acrylic/melamine coating compositions. For example, the coatings produced using carbamate-functional resins typically have excellent resistance to environmental etch (also called acid etch). Environmental etch results in spots or marks on or in the coating that often cannot be rubbed out.
One drawback of coatings with carbamate-functional resins is that they tend to require more organic solvent to achieve acceptable viscosity and for application. Carbamate-functional materials prepared from an isocyanurate of a diisocyanate, for example, are generally advantageous as an additive resin or principal resin in a coating composition, but these materials increase the viscosity of the coating composition so that more solvent is required. Coatings with higher amounts of organic solvent produce more regulated emissions during application.
The content of volatile organic compounds has also been reduced by using resins of lower viscosity, for example very low molecular weight polymeric or oligomeric materials or reactive diluent compounds. Such materials are particularly susceptible to sagging during the initial stages of baking, however, due to their rheology profile on heating. On the other hand, many agents that may be added to control rheology (so-called rheology control agents, or, in this case, anti-sagging agents) also significantly increase the viscosity of the coating composition. Hence, there is no benefit in using such rheology control agents over simply increasing the molecular weight of the reactive vehicle components.
U.S. Pat. Nos. 4,311,622, 4,677,028, and 4,851,294 describe sag control agents for coatings that are the crystalline reaction products of an amine and a diisocyanate, optionally reacted in the presence of a resinous binder. These sag control agents are crystalline, and their effectiveness depends on size and shape of the crystals as well as their concentration in the coating composition. The polymers in the coating compositions of these patents appear to be generally hydroxyl functional. Barancyk et al., U.S. Pat. No. 6,111,011, discloses polymeric coating compositions containing a rheology modifier comprising the reaction product of an amine and an isocyanate. It would be desirable, however, to have a higher solids coating composition that would not sag during application or curing.
Thus, it would be desirable to have a curable coating composition that contains carbamate-functional and/or urea-functional materials because of the superior coating properties of cured coating produced form such composition, while at the same time having lower organic solvent content in such composition without the unacceptable sagging the usually accompanies compositions made with extremely low molecular weight materials.